The satellite is the Cosmic Background Explorer (COBE). Since its Nov. 18, 1989, launch by the National Aeronautics and Space Administration, it has opened an exciting new era of exploration for cosmologists who previously had little opportunity to test their theories.

Astrophysicist Stephen Maran of the Goddard Space Flight Center in Greenbelt, Md., explains that, before COBE, cosmologists had only three observational facts to work with:

1. The universe is expanding. This is consistent with theories that the universe originated 10 billion to 20 billion years ago in the "Big Bang" explosion of a super-dense primodial mass that was smaller than the period at the end of this sentence.

2. A weak radiation of microwave and infrared energy permeates all space. Theorists consider such radiation to be the Big Bang's afterglow.

3. A measured relative abundances of light elements conform to what cosmologists expect the Big Bang would have created.

"Up to now, everything we knew about the Big Bang could be written on the head of a pin. Now we're moving beyond that," Dr. Maran observes.

COBE is equipped specifically to study key aspects of the fossil radiation.

It measures the radiation's intensity at different wavelengths from 0.5 to 10 millimeters. COBE's first major success was to show this spectrum is the same as that of a blackbody, meaning a perfect absorber and emitter of radiation, at a temperature of 2.73 kelvin (minus 454.8 degrees F).

In Big Bang theory, the universe evolving from the primeval explosion was so hot that matter particles and radiation particles (photons) freely exchanged energy. This would give the radiation a blackbody character.

After about 300,000 years, things would have cooled to the point where free interaction ceased.

However, any tendency toward an organized structure of matter should have left its imprint on the radiation in the form of slight variations in intensity.

Physicist George Smoot of the University of California's Lawrence Berkeley Laboratory electrified scientists at a meeting of the American Physical Society in Washington April 23 when he announced that the COBE team he leads has found this effect. Today, astronomers see galaxies grouped in relatively dense structures around the edges of large voids, giving the universe a fish-net or Swiss-cheese structure.

The fossil radiation shows the same structure etched in terms of regions of temperatures a mere hundred-thousandth of a degree warmer than the surroundings.

Now cosmologists can see that the structural organization around which galaxies grew was present in the very early universe.

COBE project scientist John Mather at Goddard says he expects critics to question the analysis, which is based on early data. But, he adds, "We're going to do better with [the additional data] we have now."

Finally, Dr. Mather notes, another team is working through other data looking for signs of the glow of the very first galaxies. If found, this will bring even more understanding about what went on when the early cosmos began to look like the universe we now know.